Normal males have only one X chromosome so that their chromosome count (called karyotype) is 46 XY. Males with Klinefelter Syndrome have additional X chromosome(s) so that their chromosome count most commonly is 47, XXT. Most Klinefelter Syndrome males have reproductive dysfunction such as absence of sperm, low level of male hormone called testosterone, and cognitive function (ability to learn) problems such as reading and spelling difficulties. Cognitive deficits are evident from early school years though no obvious physical features are present at that time. With increasing age both reproductive and cognitive dysfunctions tend to become more severe. Decreased bone mass is a common medical problem in adult males with Klinefelter Syndrome and testosterone is a justified treatment for adults with Klinefelter Syndrome to improve sexual function, mood, muscle strength and bone thickness. The data about testosterone action on cognition, reproductive system and bone mass in Klinefelter Syndrome boys is very sparse. There is a lack of guidelines for testosterone treatment in Klinefelter Syndrome boys. This study will determine if testosterone improves ability to learn, behavior, bone and muscle mass brain imaging pattern, bone mineral density, and bone formation markers before and after testosterone or placebo treatment in boys with Klinefelter Syndrome in the age range 10-13 years.